Audio signal recording on a phono disk is based on the well-known Recording Industry Association of America (RIAA) correction procedure. According to this procedure the amplitude of an electrical signal recorded on disk depends on the frequency. Such correction is carried out to improve the dynamic range of the signal. When the disk is played back, the electrical signal coming from the pickup cartridge to the output power amplifier and later to the speaker has to pass through a pre-amplifier with additional frequency correction, in which the reverse correction procedure is applied.
Mathematically expressed the frequency correction transmission function from the input to output of the pre-amplifier has the form:Uout/Uin=κ0*(1+i*w*b)/((1+i*w*b1)*(1+i*w*b2)),
where Uout and Uin are signal amplitudes at the output and input, respectively,
κo is frequency independent amplification factor,w=2*π*f,
f is the signal frequency,
b=318, b1=75, b2=3180 are time constants, expressed in milliseconds,
i is complex unity.
There are many different realizations of RIAA-correctors. Among them one can find transistor, solid state and vacuum tubes circuits.
Usually capacitors are implemented in such pre-amplifiers as the elements featuring frequency-dependent characteristics.
In spite of their simplicity of use, capacitors introduce the disturbances into the signal transmitted. This circumstance, however, degrades the audio characteristics of the pre-amplifier. The electrical parameters of capacitors depend essentially on the dielectrics used, the foil and the winding method. As a result capacitors possess such undesirable features as non-linearity, inductance, energetic losses during the electrical signal transmission, etc. Experts often notice the dependence of the sound on the capacitors types, the coarse sounding of cheap capacitors in the upper register of the sound signal, sticky and dim sound.
RIAA correction can be also done by means of a transformer with two secondary windings, as shown in FIG. 1. The transformer has a primary winding I and two secondary windings II-1 and II-2, which are loaded on two resistors. Electrical circuits of the type shown in FIG. 1 can provide the necessary form of amplitude vs. frequency curve of RIAA correction.
FIGS. 2a-2b show the physical construction of a transformer of the type shown in FIG. 1 in more detail. The transformer's core has a complicated form, which is expressed in different length of steel plates 1 and 3 in comparison with a central plate 2. In addition, after assembling the core, there exists additional air gap x between the central plate and a lower plate 4 (FIG. 2a).
The primary winding I is placed on the side plates 1 of the core, while the secondary winding II-2 is wound over the primary winding I, and the secondary winding II-1 is wound on a separate coil, placed on the plates 3 of the core. The position of the windings is illustrated in FIG. 2b. 
The primary winding is connected to a vacuum tube base amplifying cascade, such that in the absence of a signal at the input a constant non-zero current is present in the primary winding.
The principle of RIAA correction of the circuit in FIG. 1 is based upon two leakage inductances of the secondary windings with respect to the primary winding. The first leakage inductance of the secondary winding II-2 has a small value and is due to parasitic leakage inductance of the winding. This leakage inductance is a feature of every transformer and is well described elsewhere. Parasitic leakage inductances are usually small and depend on the number of turns in the winding and the geometry of the winding, and also on the sectioning of the windings. By changing these parameters one can obtain the necessary value of leakage inductance.
The artificial leakage inductance of winding II-1 has a large value, which can be achieved only by splitting the original magnetic flux generated by the primary winding (which passes through plate 1 of the core) into two directions: one along plate 2 with air gap x, and another along plate 3 (see FIG. 2b).
As a result, some part of the magnetic flux of the primary winding I doesn't reach the winding II-1, and is closed on the plate 2 of the core.
Such construction provides high value of the leakage inductance of the core, which is difficult to achieve by other means.
Due to the leakage inductances the high frequency part of electrical signal, transmitted from the primary winding to each of the secondary windings and resistors, is reduced, therefore the transformation factor of the signal from the primary winding to each of the resistors depends on the frequency. The ratio of smaller leakage inductance of winding II-2 to the loading resistor is given and equal to a correction time constant b1 (75 microseconds), while the ratio of higher value of leakage inductance of winding II-1 to the loading resistor is equal to a correction time constant b2 (3180 microseconds). After the summation of the signals from the two resistors, taking into account the different transformation factors from the primary to each of the secondary windings, the signal amplitude at the output of the corrector has right frequency-corrected value.
The disadvantage of this method of signal RIAA correction is that an additional magnetic bridge (plate 2 in FIG. 2a) in the transformer core is necessary to provide the leakage inductance. The additional bridge increases the size and the cost of the core.
In addition, because the primary winding is placed on the side part of the core, the cross-section area of the core is two times lower than in the case of common placement of the winding at the central part. This circumstance reduces the inductance of the primary winding, which will affect the operation of the transformer at low frequencies.
Finally, such a method doesn't allow the use of high values of leakage inductances, which are restricted by the size of the transformer. One cannot wind as many turns as one wants to obtain any value of the inductance. Therefore, the leakage inductance cannot be made very high. Because the ratio of inductance to resistor is equal to the correction time, the resistor value is also limited. This in turn implies limitations on the first driver connected to the primary winding of the transformer, which has to have an output impedance that is much smaller than the driver's load.